I ride a Honda Rebel 250 motorcycle and over the past few days I find that the bike is losing power when accelerating fast.

When I twist the throttle to accelerate quickly (in any gear) I hear the engine sound increasing in pitch by a lot more than I'm used to hearing, but the motorcycle doesn't seem to respond and performance is very poor. If I then twist it back to a position of slow acceleration, the engine noise returns to I'm used to hearing (still the accelerating noise but a lot lower pitch) and the performance also seems to be appropriate to that level of acceleration.

I am having the same issue, I have a 125cc ajs regal raptor, and if I am accelerating in well its mainly 4th gear and going up a slight incline it'll lose power and even if I knock it down into 3rd it'll struggle, but I think I may have a block in the air fliter or in the carbs, so it needs sorting, just something to think about next time you have a similer problem mate
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user3632Sep 3 '13 at 20:25

@AdamRobinson the key here is that the RPMs increase without the motorcycle accelerating. This is the hallmark of a slipping clutch, and could not be explained by blocked air filter or carb issues.
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macSep 4 '13 at 15:49